marco_b

This is a plugin / widget, after installing you have the possibility to locate a widget to your site. An visitor can now send a sms to a person.

SMSPlug

With this plugin you can use the service from aspsms.com. It allows you to send sms via their service.

Julian Yanover

Admin Msg Board is a plugin for WordPress that will add a messaging system in your WordPress admin area. This works for weblogs that are written by more than one blogger, so they can communicate with each other, without leaving the WP system. 
Messages can have up to 140 words like in twitter, and you can choose whether to write to all users or select recipients

Admin Msg Board

Adds a messaging system in your Wordpress admin area for bloggers to communicate.

SMSPlug vs Admin Msg Board